double-slit experiment

An experiment demonstrating that light and matter can satisfy the seemingly incongruous classical definitions for both waves and particles, regarded as evidence for the fundamentally probabilistic nature of quantum mechanics. A coherent light source, such as a laser beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate, where the wave nature of light causes bright and dark bands due to interference.